The production of synthesis gas by either partial oxidation or steam reforming is well known and there are extensive literature references to these processes. Each process may be used separately to produce synthesis gas or the processes may be combined.
The steam reforming reaction is highly endothermic and is described as: EQU CH.sub.4 +H.sub.2 O.revreaction.CO+3 H.sub.2 (1)
The partial oxidation reaction is highly exothermic and is described as: EQU CH.sub.4 +.fwdarw.CO+H.sub.2 +H.sub.2 O (2)
The combination of the two reactions is somewhat exothermic and is described as: EQU 2 CH.sub.4 +O.sub.2 .fwdarw.2 CO+4H.sub.2 (5)
In addition to these two primary reactions, the water gas shift reaction also occurs: EQU CO+H.sub.2 O.revreaction.H.sub.2 +CO.sub.2 (6)
The equation for the combined process shows that the ratio of produced hydrogen to carbon monoxide is 2/1. This ratio is convenient for utilizing the synthesis gas in the production of higher molecular weight hydrocarbons through the Fischer-Tropsch reaction. The ratio of H.sub.2 /CO can be varied by changing the mole ratios of CH.sub.4 /H.sub.2 O/O.sub.2 /CO.sub.2 and by changing reactor temperature and pressure.
Patents illustrating the related processes are: U.K. 637,776; 1,359,877 and 2,119,276: U.S. Pat. Nos. 3,976,504; 3,984,210; 4,048,091; 4,414,140; 4,415,484; 2,425,754; 2,541,657; 2,577,563; 2,607,670; 2,631,094; 2,665,199; 3,168,386; 3,355,248; 3,379,504; 3,573,224; 3,644,100; 3,737,291; 3,953,356; and 4,309,198; and Japanese 580,910,022. Patents illustrative of the reaction between methane, steam and oxygen over a nickel catalyst are: 1,711,036; 3,138,438; 2,467,966; 1,736,065; and 1,960,912. These latter patents show the reaction occurring either in a fixed bed of catalyst or in a fluid catalyst bed.
Catalysts comprising nickel on alumina are well known, c.f. G. W. Bridger "Catalysis", The Steam Reforming of Hydrocarbons, page 39. One conclusion of this reference is that increasing amounts of nickel up to about 20 wt % nickel increases the activity of the nickel/alumina catalyst and nickel levels for steam reforming catalyst are reported, generally, as ranging from about 8 wt % to 20 wt % of catalyst.
Fluid bed processes are well known for the advantages they provide in heat transfer and mass transfer. Fluidized beds allow for substantially isothermal reactor conditions and are effective in eliminating temperature runaways or hot spots. Such processes are not without their disadvantages and the use of fluidized systems must take into consideration the strength of the catalyst (its resistance to attrition) as well as the erosivity of the catalyst (the tendency of the catalyst to erode equipment).
The production of synthesis gas in fluid bed processes is particularly sensitive to catalyst strength. If the catalyst undergoes severe attrition in which fine catalyst particles are formed, these fine particles will be entrained in the product gas and carried through and out of the fluid bed regardless of whether cyclones are employed for fines recovery. This not only causes high catalyst make-up rates, but also causes downstream fouling and other associated problems which are further described below.
Methane conversion to synthesis gas is affected deleteriously by catalyst attrition because (i) particles are lost from the fluid bed system as fines, thereby decreasing the volumetric activity of the fluid bed of catalyst and requiring that fresh catalyst be added in order to maintain acceptable levels of methane conversion, and (ii) catalyst fines entrained in the product gas and carried out of the fluid bed tend to deposit on equipment outside the fluid bed and (because equation (1) above is reversible) methane can be re-formed as the temperature is decreased and a new reaction equilibrium is established. In either case, the net conversion of methane is decreased. Relatively high residual methane in the synthesis gas can cause a substantial economic debit when the synthesis gas is used, for example, in Fischer-Tropsch processes.
The object of this invention is not only to provide a catalytic, fluid bed, hydrocarbon conversion process that minimizes the necessity of adding catalyst to the fluid bed to maintain volumetric activity, but more importantly, to minimize the methane concentration in the cooled recovered gases by minimizing the amount of catalytic material carried out of the bed, thereby minimizing the back reaction of carbon monoxide and hydrogen to form methane.